Isoquinolin-5-amine

The title compound, C₉H₈N₂, presents two almost identical independent mol­ecules in the asymmetric unit, both of them exhibiting an extremely planar isoquinoline core (maximum r.m.s. deviation = 0.014 Å). The most significant deviation is found in the -NH₂ groups, which present a noticeable pyramidalization around the N atom, a feature also present in related structures containing the mol­ecule as a ligand. The supra­molecular structure is based on pairs of parallel hydrogen-bonded chains formed by just one mol­ecular type each, defined by the strongest hydrogen bonds in the structure, which are of the N-H...N type. These parallel chains are linked into pairs (or strips) via weaker C-H...N hydrogen bonds. Related strips generated by the c-glide plane define two families running along [10] and [110], giving rise to an inter­esting system of inter­woven chains stabilized by a number of weaker contacts of the C-H...[pi] type.

S1. Comment

We have previously reported the synthesis and structural analysis of compounds containing aromatic amines (Atria, Astete et al., 2011; Atria, Garland & Baggio, 2011, and references therein). In most of these structures, the amine groups are in complex with lanthanide or transition metal cations, and via their ability to promote nonbonding interactions of different types and strengths (hydrogen bonds, π–π interactions etc.) they ended up being the vectors for, usually, highly stable three-dimensional supramolecular structures.

On rare occasions, crystals of the free ligands were serendipitously obtained in addition to (or instead of) the expected complexes, and the analyses of their crystal structures were often worth discussing, not necessarily for the extractable molecular information (which was almost predictable beforehand) but rather for the extremely interesting three-dimensional networks they gave rise to. A representative case with diaminopurine can be found in Atria et al. (2010).

This report presents a further example of such an unexpected situation (see Experimental for details): the crystal structure of isoquinolin-5-amine, (I), a very simple molecule giving rise to an unusual supramolecular arrangement. It is worth mentioning that, in spite of its simplicity, this ligand is rare from a crystallographic point of view: only one entry in the Cambridge Structural Database (CSD, Version 5.33; Allen, 2002) could be found, with the ligand bonded to a Zn^II nucleus [bis(isoquinolin-5-amine)diazido zinc(II), Zn(N₃)₂(C₉H₈N₂)₂, (II); Miao et al., 2007].

Compound (I) crystallizes in the monoclinic space group Cc with two independent molecules in the asymmetric unit (Fig. 1), which we will characterize hereinafter by the trailing digit in their labels (1 or 2). As stated above, the molecular details do not depart either from the usual values found in the CSD or between the two molecules. A few comparative details between the two independent units are: (i) least-squares molecular fitting, overall r.m.s. deviation = 0.018 Å and maximum deviation (for the N21···N22 pair) = 0.036 Å; (ii) bond distances, overall r.m.s. deviation = 0.004 Å and maximum deviation (for the N2x—C6x bonds) = 0.009 Å (2σ); (iii) bond angles, overall r.m.s. deviation = 0.222° and maximum deviation (for the C3x—C4x—C5x angles) = 0.44° (2σ).

Both isoquinoline nuclei (1 and 2) are planar [overall r.m.s. deviation = 0.0069 Å for both molecules; maximum deviations = 0.0140 (14) Å for C21 and 0.0118 (13) Å for C22], with the amino N atoms deviating significantly from the aromatic least-squares plane [0.161 (2) Å for N21 and 0.089 (2) Å for N22]. This deviation is accompanied by a substantial (though uneven) degree of pyramidality in the arrangement around them, evidencing an N-atom hybridization with a significant sp³ contribution for N21 and a more `flattened' arrangement around atom N22, suggesting a predominant sp² character. If pyramidality is measured, as proposed by Allen et al. (1995), by χ_(N), the angle between the C—N vector and its projection into the NH₂ plane [ideal values: χ_(N) = 0° for pure sp²; 54.7° for pure sp³], the corresponding values for both N atoms in the case of (I) are χ_(N21) = 40.5° (mostly sp³) and χ_(N22) = 28.6° (very nearly midway between sp³ and sp²). This is consistent with the C21—N21 bond [1.360 (2) Å] being slightly longer than the C22—N22 bond [1.350 (2) Å], suggesting a smaller delocalization in the former. This different degree of planarity of amino groups bound to aromatic nuclei is frequently found in the literature (e.g. Atria et al., 2010) and is usually ascribed to the variable ability of the delocalized π-system of the ring to accommodate charge from the amino group in the extended resonance structure, a fact probably conditioned by environmental factors such as coordination to a metal centre or hydrogen bonding.

As expected, the main interest of (I) resides in the way in which the supramolecular structure builds up. All the responsible noncovalent interactions are of the hydrogen-bonding type, with a diversity of donors (N—H and C—H) and acceptors (N and π), all of them shown in Table 1.

The two conventional N—H···N hydrogen bonds, appearing as the first two entries in Table 1, are by far the strongest and they define the two distinct motifs in the structure (Fig. 1), viz. two parallel chains, each one formed by a single type of molecule (either 1 or 2). These are in turn linked by a weak nonconventional C—H···N interaction (third entry in Table 1), which defines the hydrogen-bonded two-chain strips shown in Fig. 2 on a grey background and running along [110] (top to bottom in the figure). The H atoms involved in the main interactions (H21A and H22A) correspond to amino groups. Surprisingly, the remaining H atoms in each NH₂ group are not involved in any type of hydrogen-bonding contact. This does not seem to be a particularly unusual effect: a search of the CSD showed that about 5% of the structures presenting aromatic amino units had their NH₂ groups asymmetrically hydrogen-bonded, as found in (I).

The one-dimensional double-chain substructures are replicated by the c-glide plane into a second family of strips (Fig. 3), forming an angle of 44.32 (2)° with the former ones but now running along [110] and represented in Fig. 2 on a white background. These chains are seen exactly in projection in the figure (with the deceptive appearance of single molecules). One of these `vertical' [110] double-chain strips has been highlighted, for clarity.

Finally, these two families are connected via C—H···π intra- and inter-strip crosslinks (entries 4–6 in Table 1), which in Fig. 2 appear as `out-of-plane' bonds with the Cg acceptors (underlined labels in the figure), which are either above or below the plane.

As a final remark, we would like to introduce here a word of caution regarding the (frequently uncritical) geometric idealization of O—H and N—H atoms, a fact which can introduce gross interpretation errors in groups like NH₂. An example can be found in the related Zn complex, (II), where this simplistic assumption was made in the published results, thus making impracticable any possible comparison with our own results. Taking advantage of the fact that the data set of (II) looked fair {R_int = 0.036 and R[F² > 2σ(F²)] = 0.029} and that the structure factors were available in the literature, we performed a new refinement of the structure of (II) with the amino H atoms subject to the same restraints as we applied for (I). To our surprise, an even more enhanced pyramidalization trend was observed around the amino N atoms in the two independent isoquinolin-5-amine units, with two strongly dissimilar degrees of sp²–sp³ hybridization and a similar tendency as in (I) in the corresponding C—N bond distances [χ: 15.9 and 40.7°; C—N: 1.358 (4) and 1.385 (3) Å]. The convenience of confirming the H-atom structure through a careful analysis of the difference maps thus becomes apparent.

Related literature

For related literature, see: Allen (2002); Allen et al. (1995); Atria et al. (2010); Atria, Astete, Garland & Baggio (2011); Atria, Garland & Baggio (2011); Miao et al. (2007).

S2. Experimental

Crystals of the title compound were obtained as a by-product during the synthesis of a family of lanthanide complexes with crotonic acid and isoquinolin-5-amine. To an aqueous solution (200 ml) containing the corresponding lanthanide oxide (1 mmol), an aqueous solution (20 ml) of crotonic acid (6 mmol) was added, followed by the isoquinolin-5-amine ligand (1 mmol) dissolved in ethanol (30 ml). The resulting mixture was refluxed for 8 h and filtered. The filtrate was allowed to evaporate at room temperature; in one of the attempts made, good single crystals of (I) suitable for X-ray analysis were unwittingly obtained.

S3. Refinement

The absolute structure could not be determined for this light-atom structure. All H atoms were clearly seen in a difference Fourier map but were treated differently in the refinement. C-bound H atoms were repositioned at their expected locations and allowed to ride, with C—H = 0.95 Å. N-bound H atoms, displaying a noticeable pyramidality, were refined with similarity restraints in the internal distances in both molecules (s.u. on N—H bonds = 0.015 Å and s.u. on H···H distances = 0.025 Å). In all cases, H atoms were treated with U_iso(H) = 1.2U_eq(parent).

Computing details

Data collection: SMART (Bruker, 2001); cell refinement: SAINT (Bruker, 2002); data reduction: SAINT (Bruker, 2002); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXTL (Sheldrick, 2008) and PLATON (Spek, 2009).

Figures

	Fig. 1. The molecular structure of (I), with displacement ellipsoids drawn at the 40% probability level. Independent (symmetry-related) atoms are shown with heavy (hollow) bonds and filled (empty) ellipsoids. Note the formation of hydrogen-bonded [110] chains. [Symmetry code: (i) x + 1/2, y - 1/2, z.]
	Fig. 2. A packing view of (I), along the [110] direction, showing on a grey background the two independent chains running top to bottom, parallel to [110], and on a white background their c-glide symmetry-related images, perpendicular to the plane of the figure (one of these strips has been isolated on a dark triangular background). N—H···N and C—H···N interactions are represented by dashed lines and C—H···π interactions are represented by thin double lines. Molecules drawn with thin lines correspond to molecule 1 and those in bold to molecule 2. Cg centroids (with underlined labels) are above or below the projection plane. [Symmetry codes: (i) x + 1/2, y - 1/2, z; (ii) x + 1/2, y + 1/2, z; (iii) x - 1/2, -y + 3/2, z + 1/2; (iv) x - 1/2, -y + 3/2, z - 1/2.]
	Fig. 3. An [001] view of two c-glide symmetry-related strips, running along the [110] and [110] directions. As in Fig. 2, molecules drawn with thin lines correspond to molecule 1 and those in bold to molecule 2. Dashed lines represent hydrogen bonds.

Isoquinolin-5-amine

Crystal data

C9H8N2	F(000) = 608
Mr = 144.17	Dx = 1.304 Mg m^−3
Monoclinic, Cc	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: C -2yc	Cell parameters from 2008 reflections
a = 6.0731 (15) Å	θ = 2.5–25.2°
b = 14.921 (4) Å	µ = 0.08 mm^−1
c = 16.285 (4) Å	T = 150 K
β = 95.686 (4)°	Block, pink
V = 1468.5 (7) Å^3	0.36 × 0.19 × 0.12 mm
Z = 8

Data collection

Bruker SMART CCD area-detector diffractometer	3082 independent reflections
Radiation source: fine-focus sealed tube	2701 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.020
CCD rotation images, thin slices scans	θmax = 27.8°, θmin = 2.5°
Absorption correction: multi-scan (SADABS in SAINT-NT; Bruker, 2002)	h = −7→7
Tmin = 0.98, Tmax = 0.99	k = −19→19
6002 measured reflections	l = −21→21

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.038	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.090	H atoms treated by a mixture of independent and constrained refinement
S = 1.04	w = 1/[σ^2(Fo^2) + (0.0471P)^2 + 0.1477P] where P = (Fo^2 + 2Fc^2)/3
3082 reflections	(Δ/σ)max < 0.001
215 parameters	Δρmax = 0.15 e Å^−3
9 restraints	Δρmin = −0.16 e Å^−3

Special details

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
N11	0.1181 (3)	0.75791 (11)	0.71764 (10)	0.0353 (4)
N21	0.4601 (3)	0.44916 (11)	0.66851 (11)	0.0350 (4)
H21A	0.506 (3)	0.3929 (10)	0.6840 (14)	0.050 (7)*
H21B	0.573 (3)	0.4876 (12)	0.6624 (13)	0.035 (6)*
C11	0.0040 (3)	0.69540 (13)	0.75160 (11)	0.0322 (4)
H11	−0.1212	0.7141	0.7778	0.039*
C21	0.0527 (3)	0.60266 (12)	0.75230 (10)	0.0268 (4)
C31	−0.0814 (3)	0.53979 (12)	0.78932 (11)	0.0310 (4)
H31	−0.2082	0.5590	0.8143	0.037*
C41	−0.0260 (3)	0.45095 (12)	0.78871 (12)	0.0339 (4)
H41	−0.1137	0.4084	0.8142	0.041*
C51	0.1587 (3)	0.42216 (12)	0.75093 (12)	0.0319 (4)
H51	0.1932	0.3601	0.7514	0.038*
C61	0.2921 (3)	0.48055 (12)	0.71302 (11)	0.0275 (4)
C71	0.2413 (3)	0.57451 (12)	0.71441 (10)	0.0255 (4)
C81	0.3646 (3)	0.64207 (13)	0.67857 (11)	0.0296 (4)
H81	0.4924	0.6266	0.6525	0.036*
C91	0.2998 (3)	0.72951 (12)	0.68152 (12)	0.0326 (4)
H91	0.3860	0.7733	0.6570	0.039*
N12	0.1848 (3)	1.14060 (11)	0.49255 (10)	0.0350 (4)
N22	0.5738 (3)	0.83554 (11)	0.53578 (11)	0.0327 (4)
H22A	0.619 (3)	0.7791 (9)	0.5251 (12)	0.027 (5)*
H22B	0.683 (3)	0.8765 (11)	0.5468 (13)	0.039 (6)*
C12	0.0535 (3)	1.07705 (13)	0.45894 (11)	0.0311 (4)
H12	−0.0870	1.0948	0.4333	0.037*
C22	0.1057 (3)	0.98471 (13)	0.45819 (10)	0.0264 (4)
C32	−0.0439 (3)	0.92095 (12)	0.42084 (11)	0.0304 (4)
H32	−0.1851	0.9390	0.3961	0.036*
C42	0.0172 (3)	0.83286 (12)	0.42081 (12)	0.0339 (4)
H42	−0.0821	0.7897	0.3952	0.041*
C52	0.2243 (3)	0.80513 (12)	0.45795 (12)	0.0317 (4)
H52	0.2616	0.7433	0.4570	0.038*
C62	0.3758 (3)	0.86487 (12)	0.49587 (10)	0.0270 (4)
C72	0.3174 (3)	0.95793 (11)	0.49562 (10)	0.0248 (4)
C82	0.4575 (3)	1.02669 (12)	0.53050 (11)	0.0299 (4)
H82	0.6009	1.0123	0.5558	0.036*
C92	0.3873 (3)	1.11346 (12)	0.52790 (12)	0.0328 (4)
H92	0.4852	1.1579	0.5522	0.039*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
N11	0.0388 (10)	0.0267 (8)	0.0405 (9)	0.0029 (7)	0.0042 (7)	0.0015 (7)
N21	0.0298 (9)	0.0296 (9)	0.0463 (10)	0.0056 (8)	0.0073 (7)	−0.0013 (8)
C11	0.0328 (10)	0.0298 (10)	0.0341 (10)	0.0052 (8)	0.0044 (8)	0.0007 (8)
C21	0.0272 (9)	0.0272 (9)	0.0251 (8)	0.0005 (7)	−0.0013 (7)	0.0007 (7)
C31	0.0277 (10)	0.0335 (10)	0.0318 (10)	−0.0005 (8)	0.0030 (8)	0.0009 (8)
C41	0.0343 (10)	0.0323 (10)	0.0349 (10)	−0.0062 (8)	0.0026 (8)	0.0048 (8)
C51	0.0356 (10)	0.0220 (9)	0.0369 (10)	−0.0003 (8)	−0.0024 (8)	0.0015 (8)
C61	0.0278 (9)	0.0260 (10)	0.0277 (9)	0.0034 (7)	−0.0023 (7)	−0.0017 (7)
C71	0.0243 (9)	0.0275 (9)	0.0236 (8)	0.0021 (7)	−0.0029 (7)	−0.0009 (7)
C81	0.0266 (9)	0.0338 (10)	0.0283 (9)	−0.0010 (7)	0.0021 (7)	−0.0002 (8)
C91	0.0353 (10)	0.0285 (10)	0.0336 (10)	−0.0044 (8)	0.0013 (8)	0.0026 (8)
N12	0.0364 (8)	0.0285 (8)	0.0397 (9)	0.0046 (7)	0.0017 (7)	−0.0021 (7)
N22	0.0291 (8)	0.0249 (9)	0.0443 (9)	0.0055 (7)	0.0045 (7)	0.0045 (7)
C12	0.0300 (9)	0.0295 (10)	0.0335 (10)	0.0056 (8)	0.0014 (8)	−0.0011 (8)
C22	0.0268 (9)	0.0290 (10)	0.0241 (9)	0.0025 (7)	0.0057 (7)	0.0005 (7)
C32	0.0257 (9)	0.0344 (10)	0.0309 (10)	−0.0016 (8)	0.0027 (8)	0.0004 (8)
C42	0.0341 (10)	0.0309 (10)	0.0372 (10)	−0.0088 (8)	0.0059 (8)	−0.0041 (8)
C52	0.0351 (11)	0.0212 (9)	0.0397 (10)	0.0003 (8)	0.0089 (8)	−0.0007 (8)
C62	0.0291 (9)	0.0250 (9)	0.0282 (9)	0.0025 (7)	0.0093 (7)	0.0021 (7)
C72	0.0277 (9)	0.0240 (9)	0.0234 (8)	0.0017 (7)	0.0059 (7)	0.0006 (7)
C82	0.0275 (9)	0.0323 (10)	0.0296 (9)	0.0001 (8)	0.0009 (7)	−0.0012 (8)
C92	0.0336 (10)	0.0290 (9)	0.0349 (10)	−0.0005 (8)	−0.0012 (8)	−0.0042 (8)

Geometric parameters (Å, º)

N11—C11	1.316 (2)	N12—C12	1.322 (3)
N11—C91	1.368 (2)	N12—C92	1.366 (3)
N21—C61	1.390 (2)	N22—C62	1.380 (2)
N21—H21A	0.913 (13)	N22—H22A	0.907 (13)
N21—H21B	0.907 (13)	N22—H22B	0.907 (13)
C11—C21	1.415 (3)	C12—C22	1.414 (3)
C11—H11	0.9500	C12—H12	0.9500
C21—C31	1.415 (3)	C22—C32	1.411 (3)
C21—C71	1.417 (2)	C22—C72	1.424 (2)
C31—C41	1.368 (2)	C32—C42	1.366 (3)
C31—H31	0.9500	C32—H32	0.9500
C41—C51	1.399 (3)	C42—C52	1.403 (3)
C41—H41	0.9500	C42—H42	0.9500
C51—C61	1.377 (3)	C52—C62	1.382 (3)
C51—H51	0.9500	C52—H52	0.9500
C61—C71	1.436 (2)	C62—C72	1.433 (2)
C71—C81	1.416 (2)	C72—C82	1.416 (2)
C81—C91	1.365 (3)	C82—C92	1.362 (3)
C81—H81	0.9500	C82—H82	0.9500
C91—H91	0.9500	C92—H92	0.9500
C11—N11—C91	116.21 (17)	C12—N12—C92	116.27 (16)
C61—N21—H21A	112.8 (14)	C62—N22—H22A	117.9 (12)
C61—N21—H21B	116.5 (13)	C62—N22—H22B	117.7 (13)
H21A—N21—H21B	113.4 (18)	H22A—N22—H22B	115.9 (17)
N11—C11—C21	125.33 (17)	N12—C12—C22	125.15 (18)
N11—C11—H11	117.3	N12—C12—H12	117.4
C21—C11—H11	117.3	C22—C12—H12	117.4
C11—C21—C31	121.54 (16)	C32—C22—C12	121.69 (16)
C11—C21—C71	117.62 (15)	C32—C22—C72	120.70 (16)
C31—C21—C71	120.84 (16)	C12—C22—C72	117.60 (16)
C41—C31—C21	119.14 (17)	C42—C32—C22	119.07 (17)
C41—C31—H31	120.4	C42—C32—H32	120.5
C21—C31—H31	120.4	C22—C32—H32	120.5
C31—C41—C51	120.61 (17)	C32—C42—C52	121.03 (17)
C31—C41—H41	119.7	C32—C42—H42	119.5
C51—C41—H41	119.7	C52—C42—H42	119.5
C61—C51—C41	122.40 (17)	C62—C52—C42	122.09 (17)
C61—C51—H51	118.8	C62—C52—H52	119.0
C41—C51—H51	118.8	C42—C52—H52	119.0
C51—C61—N21	121.04 (17)	N22—C62—C52	121.09 (17)
C51—C61—C71	118.27 (16)	N22—C62—C72	120.84 (16)
N21—C61—C71	120.47 (16)	C52—C62—C72	118.02 (16)
C81—C71—C21	116.85 (15)	C82—C72—C22	116.72 (15)
C81—C71—C61	124.42 (16)	C82—C72—C62	124.20 (16)
C21—C71—C61	118.71 (16)	C22—C72—C62	119.08 (15)
C91—C81—C71	119.98 (17)	C92—C82—C72	120.16 (17)
C91—C81—H81	120.0	C92—C82—H82	119.9
C71—C81—H81	120.0	C72—C82—H82	119.9
C81—C91—N11	123.99 (18)	C82—C92—N12	124.09 (18)
C81—C91—H91	118.0	C82—C92—H92	118.0
N11—C91—H91	118.0	N12—C92—H92	118.0

Hydrogen-bond geometry (Å, º)

Cg1–Cg3 are the centroids of the N11/C11/C21/C71/C81/C91, C21/C31/C41/C51/C61/C71 and C22/C32/C42/C52/C62/C72 rings, respectively.

D—H···A	D—H	H···A	D···A	D—H···A
N21—H21A···N11^i	0.91 (2)	2.19 (2)	3.091 (2)	179 (2)
N22—H22A···N12^i	0.91 (2)	2.18 (2)	3.083 (3)	173 (2)
C91—H91···N22	0.95	2.55	3.419 (3)	152
C92—H92···Cg1^ii	0.95	2.81	3.479 (2)	128
C31—H31···Cg3^iii	0.95	2.63	3.423 (2)	141
C32—H32···Cg2^iv	0.95	2.67	3.460 (2)	142

Symmetry codes: (i) x+1/2, y−1/2, z; (ii) x+1/2, y+1/2, z; (iii) x−1/2, −y+3/2, z+1/2; (iv) x−1/2, −y+3/2, z−1/2.